Frequency-shift keying



Sheets-Sheet 1 Filed vay 3, 1955 D D ,5m m

ATTORNEY March 1, 1960 1 H. BANNISTER ETAL 2,927,159

FREQUENCY-SHIFT KEYING 4 Sheets-Sheet 2 Filed vay 3, 1955 /NVEMTORSLAWRENCE H. BANNISTER EDWARD E. DINOWITZ DONALD M. POWERS A TTOR/VEYMarch 1, 1960 1 ,H BANN1STER ETAL 2,927,159

FREQUENCY-SHIFT KEYING 4 Sheets-Sheet 3 Filed May 3, 1955 A TTORNE YMarch 1, 1960 1 H. BANNlsTER ETAL 2,927,159

FREQUENCY-SHIFT xmms Filed May 3, 1955 4 Sheets-Sheet 4 FIG. 7 1B-wvsfvrons F| G. 5 LAWRENCE H. BANNISTER EDWARD E. DINOWITZ DONALD M.POWERS A TTUR/VEY FREQUENCY-SHIFF.` KEYlNG Lawrence H. Bannister,Hanover, Edward E. Dnowitz, Needham Heights, and Donaid M. Powers,Weston, Mass., assignors to Laboratory for Electronics, Inc., Boston,Mass., a corporation of Delaware Application May 3, 1955, Serial No.505,704

19 Claims. (Cl. 178-66) The present invention relates in general tosignalling systems and in particular to a novel system providing aplurality of frequency-shifted sub-carriers with limit frequenciescontrolled by a single crystal and receivable substantially free fromunwanted amplitude modulation.

Fundamentally, a frequency-shift keying system, commonly known as FSK,transmits information by selecting for emission one of two limitfrequencies, one of which may be called the mark frequency, and theother of which may be called the space frequency. One method ofobtaining this shift is by varying the frequency of an oscillator fromone limit frequency to the other. ln normal radio-Teletype applicationswhere only a single carrier is being keyed, this method is foundsatisfactory. However, when a number of sub-carriers are each to bekeyed between two limit frequencies, and the mark frequency of onesub-carrier is close to the space frequency of another sub-carrier, theproblem of frequency stability arises because it is necessary tomaintain these limit frequencies very close to their nominal values.lThus it became desirable to use an alternate method of keying; namely,switching between two crystalcontrolled limit frequencies.

However, this technique introduces another problem. To separate thevarious closely spaced sub-carriers, the receiver must include filtershaving very narrow pass bands. When a signal in the pass band is appliedsuddenly to the input of a filter, there is an exponential build-up ofthe signal at the output. Similarly, when a signal is removed suddenlyfrom the input of a filter, the output signal decays exponentially.Since the filter is a linear element, these effects are summed; hence,when switching from a mark to a space frequency, there is a decay of themark frequency and a build-up of the space frequency. If the twofrequencies are of equal amplitude and are in phase at the time ofswitching, there is only a slight ripple in the output of the filter.However, if they are not in phase, there is a distinct amplitude dip, oramplitude modulation. In a system utilizing amplitude modulation fortransmission Vof additional information, this dip would be intolerable.

Consequently, it is a primary object of this invention to provide amulti-channel frequency-shift keying system utilizing a plurality ofstable sub-carrier limit frequencies switched in a manner which avoidsunwanted amplitude response in the receiver.

Another object is to provide apparatus in which pertinent information istransmitted by combining amplitude modulation and frequency shift keyingof a plurality of stable carrier signals.

If it were necessary to use a separate crystal or other frequencystabilizing element for each limit frequency as in prior systems, thecomplexity and expense of utilizing a plurality of sub-carriers would beprohibitive. In accordance with the concepts of this invention, however,a plurality of subcarrier limit frequencies and a timing nited StatesPatent O ing the normal exponential rise.

ECS

frequency are all derived from a single stabilized frequency Source.

Other objects and advantages of this invention will become apparent fromthe following specification with reference to the accompanying drawingsin which:

Fig. 1 is a graphical display of the effects in the receiver filteroutput with two limit frequencies out of phase at the time of switching;

Fig. 2 is a graphical picture of the effects in the receiver filteroutput with two limit frequencies in phase at the time of switching;

Fig. 3 is a block diagram of a preferred embodiment of the invention;

Fig. 4 is a schematic circuit diagram of portions of the black diagramof Fig. 3;

Fig. 5 is a graphical presentation of a 500 c.p.s. square wave signaland a c.p.s. timing frequency signal plotted to a common time scale;

Fig. 6 is a more detailed block diagram of the countdown chain disclosedin Fig. 3;

Fig. 7 is a schematic circuit diagram of the bistable multivibratordisclosed in Fig. 6; and

Fig. 8 is a graphical presentation of the output waveform of the mixingbridge shown in Fig. 4.

' With reference now to the drawing, the nature of the problem willbecome apparent from examination of Fig. l where the signal output of ahigh Q filter is shown, assuming the filter had been energized by aconstant amplitude signal of frequency f1 long prior to a time IS, atwhich point the signal is suddenly removed. The portion of the waveformafter tS illustrates the normal exponential decay. Also shown in Fig. lis the output of the same filter, assuming it has been energized at tsby a constant amplitude signal of frequency f2, illustratlt will beobserved that the signals at frequency f1 and f2 are in exactly oppositephase at time ts. As a practical matter, when switching Vfrom one limitfrequency signal to the other, the output of a filter of an FSK receiveris the sum of two signals similar to those described above. From alinear vaddition of both signals, also shown in Fig. l, it is seen thatshortly after the instance of switching, a relatively marked amplitudefluctuation (or amplitude modulation) occurs in the region between thetwo constant amplitude time intervals. Thus, if this summation signalwere to be used for the transmission of intelligence to a remotereceiver, the amplitude transient might seriously impair thesignificance of the communication.

Fig. 2 illustrates the effect of a similar phenomenon, with theexception that at time z*s the signals at frequency f1 and f2 areprecisely in phase. It is evident from examination of the summationsignal that under these circumstances, there is substantially noamplitude modulation introduced in the resultant signal. Havingdiscovered the phase relation between the signals which virtuallyeliminates the unwanted amplitude modulation, it is appropriate toconsider methods and apparatus for practicing the invention.

Broadly speaking, the present invention provides, as an output from eachinformation channel,a signal which is switched between two fixedfrequency limits, the differencebetween which is some integral multipleof a timing `frequency. The phase of the difference frequency signalobtained by mixing the two limit frequency signals is compared with thephase of the timing frequency signal and a ignal indicative of any phasedifference between these is made to adjust the phase of one or bothlimit frequency signals until the phase difference between thedifference frequency signal `and the timing frequency signal is apredetermined value. The switching of a channel output from one limitfrequency to the other is synchronized with the timing signal so thatthe switching .occurs only when Y100 c.p.s. difference frequency, f2-f1.

3 the two limit frequency signals are substantially in phase. The timingsignal is preferably a square wave.

With reference now to Fig. 3, there is shown the block diagram of apreferred embodiment of the invention. The frequency from crystaloscillator 11 is divided in a count-down chain 12 to provide a pluralityof signal frequencies which are integral quotients of the oscillatorfrequency, one lof which Ais used ask a timing frequency. The remainingquotient signals are combined in mixers like mixers 13 to provide aplurality of pairs of limit frequency signals such that'Y the differencebetween the limit frequencies of a pair is some integral multiple of thetiming frequency. lThe amplitudes of the limit frequency signals areregulated by AGC circuits 16 to be of the same predetermined value.Hereafter, the discussion will be limited to one pair of limit frequencysignals,l f1 and f2, it being understood that the other channels areoperative in the same manner. The two 'limit frequency signals arecombined in mixer 17 after filtering'lby filters'14 and 15, providing adifference frequency output fz-fl whose phase is compared in phasedetector 18 with the timing frequency signal, or one of its harmonics,from output 21 of the count-down chain 12. The output of phase detector18 is applied to the phase-control input of filter 15, varying the phaseof f2 until f2-f1 is substantially in phase quadrature with Vthefundamental component of the square wave timing signal. This conditionrequires f1 and f2 to have the same phase at the switching time. Thelimit frequency signals f1 and f2 are applied subsequently to switch 19.The timing signal is applied to coder 22 which encodes the, informationinput into a pulsed output synchronized with the timing signal. Thepulsed output is applied to switch 19, switching output 23 betweenoutputs 24 and 25 only when f1 and the phase-corrected f2 aresubstantially in phase. 'Ihe resulting message provided by output 23 maybe amplitude modulated with additional information and when applied to areceiver filter, it`wil1 provide no undesired amplitude modulation atthe output.

' The foregoing discussion of Fig. 3 should facilitate the understandingof Fig. 4 wherein the schematic circuit diagram of filters 14 and 15,AGC circuits 16, mixer 17, phase detector 18, and switch 19 are shown.Wherever applicable, the reference numerals of Fig. 3 are carried overin Fig. 4.

- With reference now to Fig. 4, two signals, respectively having acomponent at the frequency of limit signals, f1 and f2, enter atterminals 26 and 27 respectively. Since filters 14 land 15 aresubstantially identical in operation with the exception that filter hasthe phase control, only filter 15 will be described in detail.

rsignal fZ-fl occurs at a time coincident with an edge of the squarewave timing signal. This will be discussed in detail when the operationof phase detector 18 is explained.

The signal of frequency f2 across tuned circuit 32 is amplified by V3and thence -applied to the grid of cathode-follower V4. The output ofcathode follower V4 is applied to one grid 35 Yof the mixer V5 bycapacitor 37 and resistor 36 for mixing with the signal of frequency f1from filter 14, the latter being applied to the grid 38 of V5. Thesignal on the plate of V5 includes the irIhe high fre- Vstant in timewhen f1 and f2 are in phase.

of reactance tube V2 yas a control voltage.

quency signals are bypassed largely by capacitor 41 and the 100 c.p.s.difference frequency is applied to point X of the diode bridge 42comprising V6, V7, V8 and V9 through resistor-capacitor coupling network43.

The Ysquare-wave timing signal, of frequency chosen to be 100 c.p.s. inthis embodiment, is applied symmetrically from input terminals 44 tobridge 42-through resistorcapacitor coupling networks 46 and is used asa bridge switching signal. Fig. 8 is the Waveform on point X of thebridge 42 resulting from the mixing of the square wave timing frequencysignal and the 100 c.p.s. sinusoidal difference-frequency signal fromthe plate'of mixer tube V5. The angle q is a measure of the differencebetween theV time the sinusoidal waveform reaches its maximum and thesquare wavepolarity changes. In radian measure g5 is Ede Hence, there isno D.C. component when the maximum amplitude of the sinusoidal waveformcoincides with an edge of the square wave. The maximum amplitude of thedifference-frequency waveform corresponds to an in- Y By switching fromone `frequency to the other at one delined edge Aof the timing frequencysignal the transient free switching described above is attained. Thedefined edge of the timing frequency signal will be considered below inthe discussion of Figs. 5 and 6. Accordingly, the D.C. component of thewaveform at point X appears at the output of low-pass filter 45 and isapplied to the grid The phase of the signal of limit frequency f2 iscontrolled thereby to maintain point X at substantially zero D.C.potential.- t

The phase control described above,'which will correct for verrors of iis adequate when all the frequencies to be mixed are derived from thereference source of fixed frequency by dividing repeatedly by two.However, additional phase correction is required when some of the mixingsignal frequencies are odd multiples of the timing frequency.

With reference now to Fig. 5, there is shown the vc.p.s. square wavetiming signal plotted graphically .to

the same timescale as the 500 c.p.s. square wave available fromcount-down chain-12 in this embodiment. It was stated above that theswitching betweenv limit frequency signals occurred in synchronism witha defined edge of the timing signal.V The defined edge of the squarewave timing signal corresponds ,to an instant in timewhen theY output atterminal F of multivibrator 67 of Fig. 6 suddenly changes to a lesspositive potential. The waveform plotted in Fig. 5a is that whichappears on the Vaforementioned vterminal while that of Fig.I 5b appearson terminal. F of ymultivibrator 73 in Fig. 6. The terminals discussedabove will be described in the discussion below of Fig. 6 and Fig. 7. lfthe 5,00 c.p.s. square wave shown in Fig. 5b goes positive lwhenfthetiming signal goes negative, then without phase control a limitfrequency signal which uses the 500 c.p.s. signal in its derivation maybe more than 90 In' such a case, application of a phase conessaies trolvoltage from phase detector i8 will fail to makev the phase differencebetween the two limit signals zero at the appropriate time.

At time to, it is apparent that the 500 c.p.s. signal is positive goingwhen the 1GO c.p.s. signal is negative going. When this occurs, the 500c.p.s. signal is caused to change again almost immediately thereafter.'Ihis very short time interval is indicated at At in Fig. 5. Thereafter,when the 100 c.p.s. signal goes negative, so does the 500 c.p.s. signalas illustrated at time t1. The means for accomplishing this is descriedbelow.

With reference now to Fig. 6, there is shown a detailed block diagram ofcount-down chain 12. The explanation of this diagram will clearly showthe means by which the additional phase control discussed above isaccomplished.

The schematic circuit diagram of a conventional bistable multivibratorused in the -squares of Fig. 6 is shown in Fig. 7 to facilitate theunderstanding of Fig. 6. However, since the operation of this circuit iswell known, it will not be described in detail. lt should suffice to saythat trigger pulses of appropriate polarity are applied to yeither inputterminal C, D, G, or combinations thereof, -depending on the particularapplication of the circuit. `yOutput signals are available at either orboth of termi- :nals E and F, again dependent on the particular applica-:tion.

In the discussion which follows a specific value of oscillator frequencydivided a specie number of times is :mentioned for illustrative purposesonly. Any frequency may be divided any number of times without departingfrom the teachings disclosed herein.

Returning to Fig. 6, the 128 kc. signal from crystal oscillator l1 isdivided 16 times by muitivibrators 51 through 54. The 8 kc. square waveat output F of multivibrator 54 is used to trigger quinary counter 55and multivibrator 56. Ordinarily, multivibrators 57, 61 and 62 wouldstore eight pulses before providing an output pulse. By utilizing theoutput of multivibrator 63 to reset multivibrators 57 and 61 to a countof three instead of zero, the four multivibrators in combination provideone negative going pulse for every tive negative going input pulses.Multivibrator 63 provides negative going pulses from its E output,1/16000 second after multivibrator 54 provides a pulse at its F outputterminal because the C input of multivibrator 63 is triggered by the Eoutput of multivibrator 54. This prevents muitivibrators 57 and 61 fromreceiving two trigger pulses simultaneously.

The 1600 c.p.s. square wave from multivibrator 62 is divided bymultivibrator 64 to provide an 800 c.p.s. square wave. The 80() c.p.s.signal is divided 8 times by cascaded multivibrators 65, 66, and 67,providing the 10() c.p.s. timing signal at terminals E and F ofmultivibrator 67.

Meanwhile, multivibrators 56, 71 and 72 functioning as dividers providea like square wave output at terminals E and F of multivibrator 72.Further division by multivibrator 73 provides 500 c.p.s. square waves atits E and F output terminals.

The multivibrator 74 is triggered by the negative going edges of thetiming frequency square wave at terminal F of multivibrator 67. However,a negative going edge does not appear at terminal E of multivibrator 74until input C thereof is energized by a negative going edge fromterminal E of multivibrator 54. The negative going edge from terminals Eof multivibrator 74 is applied to terminals G of multivibrators 56, 71,72, and 73 at a time 54,6000 second after their being triggered atterminals C and D. This prevents two trigger pulses from being appliedto a particular multivibrator simultaneously. These negative goingpulses on a terminal G actuate only "a multivibrator where the precedingpolarity shift in the output waveformfon terminal F was positive going.This ,maintains the desired phase relationsdescribed above.

Continuing with the discussion of Fig. 4, the signals of limitfrequencies f1 and f2 from cathode followers V6 and V4 respectively arecoupled to the grids of the switching tubes Viti and Vil respectively.Resistors 47 provide isolation between the source of switching voltageapplied at terminals A and B and the sources of limit frequency signals.

The gating signals applied at A and B are identical in waveform butopposite in polarity. This signal is applied from coder 22 andsimultaneously causes one of the switching tubes to cut off whilecausing the other to conduct. The gating signals from coder 22 aresynchronized with the timing signal so that they simultaneously shiftfrom one polarity to the other in synchronism with the defined edge ofthe square-wave timing signal. The output terminal O is switched therebyfrom one L'mit frequency signal to the other only when the two signalsare simultaneously in phase. The low pass lter 4S prevents any highfrequency transients resulting from the switching operation fromreaching terminal O. The output at terminal O is then a constantamplitude waveform of the type shown in Fig. 2.

The coder 22 which provides the gating waveform may select either limitfrequency of a channel for any time interval that is an integralmultiple of the timing signal period. It makes no difference whether thelimit frequencies represent the mark and space of a radio Teletypesystem or the zero and one of a coded binary number information system.

By adding another phasecorrection loop to control the phase changeintroduced by the filter 14 the range over which the described phasecontrol system operates can be increased. This permits higher Q resonantcircuits to be used which in turn provides better filtering whenunwanted modulation products in the output of a mixer are relativelyclose to a desired limit frequency.

The above system provides a plurality of channels with dualcrystal-controlled limited frequencies and a crystalcontrolled timingfrequency, all derived from and controlled by a single crystal. By thisnovel method of selecting the switching time, the receiver selectivefilters provide an output free from undesired amplitude transients.

It is apparent that one skilled in the art may now make numerousmodications of the particular apparatus described herein withoutdeparting from the spirit of the material disclosed herein and so theinvention is to be construed as limited only by the spirit and scope ofthe appended claims.

What is claimed is:

1. Frequency shift keying apparatus comprising a source of a timingfrequency signal and two fixed frequency signals separated in frequencyby an integral multiple of said timing frequency whereby said fixedfrequency signals are substantially in phase at times separated by theperiod of said timing frequency signal and means responsive to saidtiming frequency signal for switching between said fixed frequencysignals only when they are substantially in phase.

2. In a frequency shift keying system apparatus comprising a source of atiming frequency signal and two fixed frequency signals each frequencythereof and the difference therebetween being integral multiples of saidtiming frequency, and means responsive to said timing frequency signalfor providing one of said fixed frequency signals as an output signalwhich is changed to the other only when both signals are substantiallyin phase.

3. Frequency shift keying apparatus comprising a source of a timingfrequency signal and two fixed frequency signals separated in frequencyby said timing frequency whereby said fixed frequency signals aresubstantially in phase at times separated by the period of said timingfrequency signal and means responsive to said timing frequency signalfor switching between said 7 fixed frequency signals only when they aresubstantially in phase. l 4. Frequency shift keying apparatus comprisinga source of a timingfrequency signal and two equal amplitudexed'frequency signals separated in frequency by said timingV -frequencywhereby said iixedfrequency signals are substantially in `phase at timesseparated by the period of said timing frequency signal and meansresponsive to said timing frequency signal for switching between saidxed frequency signals only when they are substantially in phase.

5. In a frequency shift keying system apparatus comprising a source of atiming -frequency signal and two fixed frequency signals each frequencythereof harmonically related to and separated by said timing frequencywhereby said fixed frequency signals are substantially in phase at timesseparated by the period of said timing frequency signal, and meansresponsive to said timing frequency signal for providing one of saidfixed frequency signals as an output signal which is changed to theother only when both are substantially in phase.

6. In a frequency shift keying system apparatus comprising a source of atiming frequency signal and two equal amplitude fixed frequency signalseach frequency thereof harmonically related to and separated by saidtiming frequency whereby said fixed frequency signals are substantiallyin phase at times separated by the periodV of said timing frequencysignal, and means responsive to said timing frequency signal forproviding one of said fixed frequency signals as an output signal whichis changed tothe other only when both are substantially in phase. t

7.*Frequency shift keying apparatus comprising, a

source of a timing frequency signal and two Vfixed frequency signalsseparated in frequency by said timing frequency, a mixer for combiningsaid fixed frequency signals to provide a difference frequency signal, aphase comparator which provides a phasing signal by comparing saidtiming signal and said difference frequency signal, means forcontrolling the phase difference between said `fixed frequency signalswith said phasing signal to be substantially zero coincident withperiodic selected alternate polarity reversals of said timing signal,'and selective switching means for providing only one of said fixedfrequency signals as an output signal which is changed to the other onlycoincident with a selected polarity reversal.

8.4 Frequency shift keying apparatus comprising, a source of a timingfrequency signal and two equal amplitude fixed frequency signalsseparated in frequency by said timing frequency, a mixer for combiningsaid fixed frequency signals to provide a difference frequency signal,

a phase comparator which provides a phasing signal by comparing saidtiming signal and said dierence frequen- `cy signal, means forcontrolling the phase difference be.

tween said fixed frequency signals with said phasing signal to besubstantially zero coincident with periodic selected alternate polarityreversals Vof said timing signal, and selective switching means forproviding only one of said fixed frequency signals as an output signalwhich isA changed to the other only coincident with a selected polarityreversal.

9.V In a frequency shift keying system apparatus comprising, a source ofa timing frequency signal and a pair of equal amplitude fixed frequencysignals each frequen- Vcy thereof harmonically related to and separatedby said timing frequency, a mixer for combining said fixed frequencysignals to provide a difference frequencyrsignal, a phase comparatorwhich provides a phasing signal by comparing the phase-ofY said timingsignal with that ofsaid difference frequency signal, means forcontrolling the phase difference between said fixed frequency signalswith, said phasing signal to be substantially zero coincident withperiodic selected alternate polarity reversals of said timing signal,and selective switching means whichlA `selects one of said fixedfrequency signals as an output signal which is changed'to the other onlycoincident with a selected polarity reversal. Y

10. In a multi-channel frequency-shift keying system apparatuscomprising a source of a fixed-'frequency signal, means for derivingtherefrom a timing signal and a plurality of pairs of limit frequencysignals, means for selectively providing one of said limit frequencysignals from each pair as an output signal, and means responsive to saidtiming signal for changing the output signal from a pair to the otherlimit frequency only when the limit frequencies of said pair aresubstantiallyin phase.

1l. Frequency-shift keying apparatus comprising, a source of first andsecond fixed frequency signals, means for controlling the amplitude ofsaidfixed frequency signals to be a predetermined value, a source of atiming signal of frequency which is the difference between said firstand second fixed frequencies, means for mixing said first and secondfixed frequency signals to provide a difference frequency signal, aphase detector energized by said timing signal and said differencefrequency signal which provides an output for controlling the phase ofsaid second fixed frequency signal to maintain a predetermined phaserelation between said difference frequency signal and said timingsignal, and switching means operative in synchronism with said timingsignal for providing one of said fixed frequency signals as an outputsignal and changing to the other only when both are substantially inphase.

l2. Multi-channel frequency-shift keying apparatus comprising, a sourceof a fixed frequency signal, means for integrally dividing said fixedfrequencyy signal to provide a plurality of quotient signals and onetiming signal of a predetermined frequency, means for mixing saidquotient signals to provide a plurality of pairs of first and secondlimit frequency signals whose frequency difference equals the frequencyof said timing signal, means for controlling the amplitude of said limitfrequency signals to be a predetermined value, means for mixing said rstand second limit frequency signals of a pair to provide a differencefrequency signal, a phase detector for each pair which provides anoutput for controlling the phase of said second limit frequency tomaintain a predetermined phase relation between said timing signal andsaid difference frequency signal, switching means associated witheachpair operative in synchronism with said timing signal for selectivelyproviding one of said limit frequency signals in each pair as an outputsignal and changing to the other only when both are substantially inphase.

13. In a multi-channel frequency shift keying system apparatus for eachchannel comprising, a pair of mixers, a source of fixed Vfrequencysignals, a source of a timing signal, each mixer energized by at leasttwo of said xed frequency signals, a pair of filters, each with a phasecontrol input and an amplitude control input, the one filter energizedby the one mixer, the other filter energized by the other mixer, asource of an amplitude reference signal, an automatic gain controlcircuit associated with each filter for comparing the output signal ofeach filter with said amplitude reference signal to provide an amplitudecontrol signal, the amplitude control input of each lter energized bysaid amplitude control signal provided by the associated automatic gaincontrol circuit, means for combining the output signals of each filterto provide a difference frequency signal whose frequency equals that ofsaid timing signal, a phase detector associated with each lter forproviding as an output a phase control `signal indicative of the phasedifference between said difference frequency signal and said timingsignal, means .for applying said phase control signal to the phasecontrol input of the associated filter, and switching means responsiveto said timing signal for providing as a final output signal only one ofsaid filter outputV signals changeable to the other only when bothfilter output signals are substantially in phase.

14. in an information transmission system for conveying selected binarybits spaced in time by a bit period, frequency-shift keying apparatuscomprising, a source of a timing frequency signal having selectedperiodically spaced polarity reversals whose period is said bit period,a source of Zero and One xed frequency signals respectivelycharacteristic of the binary digits Zero and One, each frequency thereofand the frequency dierence therebetween being integral multiples of saidtiming frequency, a mixer for combining said Zero and One signals toprovide a difference frequency signal, a phase detector for comparingthe phase of said difference frequency signal with said timing frequencysignal to provide a phasing signal, phase control means energized bysaid phasing signal for constraining the phase dierence between said Oneand Zero signals to be substantially zero coincidentally with saidselected periodically-spaced polarity reversals, and means for selectingthe fixed frequency signals characteristic of the selected bit to beconveyed and changing therebetween only coincident with a selectedpolarity reversal.

15. Frequency shift keying apparatus comprising, a source of a stablefixed frequency signal, a plurality of serially connected flip-flops fordividing said stable signal into a high frequency square wave signalwhich has leading and trailing polarity reversals during each periodthereof, a second plurality of serially connected iphops set and resetby a polarity reversal of the preceding stage, the first stage being setand reset by said leading polarity reversals, thereby dividing said highfrequency square wave signal into a plurality of lower frequency squarewave signals, means for dividing said high frequency square wave signalto provide an odd square wave signal whose frequency is an oddsubmultiple of that of said rst square wave signal, a third plurality offlip-hops for dividing said odd square wave signal to provide a timingsquare wave signal which has selected polarity reversals alternatingwith unselected polarity reversals, the time interval between alternatepolarity reversals defining a bit period, a phasing ipflop set by saidselected polarity reversal and reset by said trailing polarity reversalof said first square wave signal which when reset provides a phasecorrecting signal for resetting said second plurality of hip-flopsthereby providing a plurality of square wave signals whose polarityreversals which coincide with said selected polarity reversals are ofthe same sense, means for combining pluralities of said square wavesignals to provide a pair of first and second fixed frequency signalsthe frequency dierence therebetween being equal to said timingfrequency, first and second filters energized by said pair for providingsaid first and second fixed frequency signals respectively as an output,a mixer which combines said first and second fixed frequency signals toderive a dierence frequency signal, a phase comparator for comparing thephase of said timing signal with said difference frequency signal toderive a phasing signal, means for varying the phase shiftcharacteristic of said first filter to maintain substantially norelative phase difference between said rst and second fixed frequencysignals coincident with said selected polarity reversals, a source of areference signal, an amplitude comparator for each filter for comparingthe output signal amplitude with said reference signal to provide a gaincontrol signal, means for applying said gain control signal to anassociated filter thereby maintaining the amplitude of each filteroutput signal at substantially the same constant level, and switchingmeans which selects said rst or second xed frequency signal as an outputand changes therebetween only coincident with a selected polarityreversal to provide effectively a frequency shifted signal substantiallyfree from undesired switching-induced amplitude variations.

16. In a multi-channel frequency shift keying system, apparatuscomprising, a source of fixed frequency signals, a pair of mixers, eachenergized by said source of fixed frequency signals and having a set ofoutput terminals, a filter associated with each mixer having an inputterminal set, an output terminal set, a phase control terminal set, andan amplitude control terminal set, means for coupling the outputterminal set of a mixer to the input terminal set of the associatedfilter, automatic gain control means having an output terminal set, aninput terminal set and an amplitude reference terminal set associatedwith each filter, a source of an amplitude reference signal, means forcoupling said source of amplitude reference signal to said amplitudereference input terminal set, means for coupling the output terminal setof said automatic gain control means to said amplitude control terminalset, means for coupling the output terminal set of said lter to theinput terminal set of said automatic gains control means, a final mixerwith an input terminal set coupled to the output terminal set of eachfilter and having an output terminal set, a source of a timing signalhaving selected polarity reversals alternating with unselected polarityreversals, a phase detector with one set of input terminals coupled tothe output terminal set of said final mixer, the other input terminalset coupled to said source of a timing signal, and its output terminalset coupled to the phase control terminal input set of one of saidfilters, an output` terminal set, and switching means which couples saidoutput terminal set to one of said filter output terminal sets andchanges to the other only in synchronism with said selected polarityreversals.

17. In a signalling system apparatus comprising, a source of a pluralityof fixed frequency signals, means for providing a first of said fixedfrequency signals as an output signal, means for deriving a signalindicative of the phase relation between said first signal and a secondof said fixed frequency signals, and switching means responsive to thephase indicative signal for changing the output signal to said secondfixed frequency signal only when said first and second signals aresubstantially in phase.

18. Signalling apparatus comprising, a source of a plurality of fixedfrequencysignals and a timing signal, and switching means which providesa first of said xed frequency signals as an output signal and isoperable in response to said timing signal for selectively substitutinga second of said fixed frequency signals as an output only when saidfirst and second signals are substantially in phase.

19. Frequency shift keying apparatus comprising a source of a timingsignal and two fixed frequency signals, means responsive to said timingsignal for effecting substantially zero phase difference between saidtwo fixed frequency signals coincident with selected portions of saidtiming signal, and selective switching means operable in response tosaid timing signal for providing only one of said fixed frequencysignals as an output signal which is changed to the other onlycoincident with said timing signal selected portions.

References Cited in the file of this patent UNITED STATES PATENTS

